1. Field of the Invention
This invention relates to an electrically assisted vehicle such as an electrically assisted bicycle or wheelchair having a human drive system for driving a driving wheel by applying a human driving force and an electric drive system for auxiliarily driving the driving wheel by driving an electric motor in correspondence with the human driving force.
2. Description of the Related Art
Generally known electrically assisted vehicles of this kind include vehicles which reduce the amount of human driving force needed to drive them by supplementing the human driving force with an electric driving force from an electric motor.
In such electrically assisted vehicles, safety during high speed traveling has been ensured by controlling an assist ratio as shown in FIG. 7, for example. That is, a kind of control has been carried out which sets the ratio of the electric driving force to the human driving force, or so-called assist ratio, to be 1.0 at traveling speeds of 0 to 15 km/h, varies it so that it gradually decreases linearly from 1.0 to 0 with speed increase between 15 and 24 km/h and sets it to be 0 at 24 km/h and over, thereby stopping the auxiliary drive provided by the electric driving force.
To achieve this, normally, a method has been adopted wherein a driving force sensor for detecting the amount of the human driving force applied to the driving wheel, a speed sensor for detecting the traveling speed and a microcomputer for calculating how large the electric driving force is to be made are provided, and the amount of the human driving force and the traveling speed are inputted in the microcomputer which is allowed to vary the assist ratio on the basis of table data stocked therein.
For example, supposing that a human driving force of 100 kg.multidot.cm is acting on the driving force sensor, an auxiliary additional output provided by the electric driving force is set to be 100 kg.multidot.cm when a traveling speed is 10 km/h(see FIG. 7), while it is calculated and outputted to be 44 kg.multidot.cm for a traveling speed of 20 km/h because the assist ratio must be reduced to about 0.44.
However, with the kind of construction described above, it has happened that when the speed sensor is inaccurate or the speed sensor malfunctions, the speed reported by the speed sensor and the actual traveling speed are sometimes different, so that, although normal calculation is carried out in the microcomputer, the assist ratio goes outside the limit of control shown in FIG. 7. Thus, it may sometimes become a dangerous travel or result in an insufficiency of electric driving force assistance.
Also, because the speed sensor detects the traveling speed from a wheel or the like which is a rotating part, there has been a problem that wiring is necessary and the construction is liable to become complicated and the wiring is liable to constitute an obstruction.